Japanese Laid-open Utility Model 4-134913 discloses a drill screw comprising a head portion, a threaded portion adjacent the head portion, a drill shaft portion adjacent the threaded portion, a large diameter drill portion adjacent the drill shaft portion, a small diameter drill shaft portion adjacent the large diameter drill portion and having a smaller diameter than the drill shaft portion, and a small diameter drill portion adjacent the small diameter drill shaft portion. However, the drill angle of the small diameter drill portion is larger than the drill angle of the large diameter drill portion so that the bite of the screw on the plate is poor. Skidding and drift are likely to occur as drilling is initiated thus making it difficult to drill an accurately placed hole. Furthermore, since the small diameter drill shaft portion is located between the large and small diameter drill portions, a longer stroke is required to obtain penetration, the work load is larger, and hence the working efficiency is poor.
Japanese Laid-open Utility Model 5-25013 discloses a drill screw comprising a head portion, a threaded portion adjacent the head portion, a drill shaft portion adjacent the threaded portion, a large diameter drill portion adjacent the drill shaft portion, and a small diameter drill portion adjacent the large diameter drill portion. The drill angle of the small diameter drill portion in this device is also larger than the drill angle of the large diameter drill portion so that it too exhibits a poor tendency to bite into the plate and is subject to skid and drift.
Because of their poor biting characteristic and tendency to skid and drift, a worker must apply a large thrusting force thus making both of the prior art devices extremely difficult to use when the fastening is to be made at a location higher than the head of the worker.